Most high power diode lasers are edge emitting semiconductors, with the laser emitted from one facet. Typical dimensions of one diode laser are a facet of 500 microns wide and 100 microns high with a length between 0.6 mm to 3 mm. The optical brightness of the diode laser is defined by its internal structure. Parallel to the pn-junction (fast axis) the emission is diffraction limited and emerging from an aperture of about 1 micron with a divergence of typical 30 degrees (half angle for 1/e2), in a direction parallel to the mounting surface of the diode laser the light is emerging from an aperture with a typical size in the range of several microns to 200 micron and a typical divergence of 7 degrees (half angle for 1/e2).
High power diode lasers with output powers of 100 W and more are realized by arranging multiple edge emitting diode lasers, so called single chips, next to each other in one semiconductor, so called array. In such cases, special measures have to be taken to provide for an efficient dissipation of heat generated by the multiple diode lasers.
Focusing the light of edge emitting diode lasers to a small spot requires optical elements for collimation and focusing. However, it is noted that the beam quality of such a high power laser diode is highly asymmetric. Typically, in fast axis the beam quality is diffraction limited in fast axis, characterized by M2=1 and in slow axis the beam quality for a 100 micron broad aperture is in the range of M2=24. The issue of asymmetric beam quality in fast and slow axis is much more severe for high power arrays, with multiple single chips arranged in one semiconductor next to each other in slow axis direction. At a beam quality of M2=1 in fast axis, the beam quality in slow axis can decrease to M2=1.000. Because the output laser beam is highly asymmetric, typically two collimating steps are performed in the prior art. Typically first micro-optical lenses are used for collimating the highly divergent beam in fast axis and secondly, collimating lenses for slow axis collimation are deployed resulting in a beam collimated in both axis that can subsequently focused with one or more lenses to a small spot. Nevertheless, the symmetry of said collimated output laser beams is not satisfactory for many applications. Accordingly, there exists a need to provide simple and cost-efficient solutions to enable high power laser diodes to output laser beams of high beam quality, in particular high symmetry.
For coupling the light into optical fibers, which is a preferred application of the subject-matter of the present application, the beam quality must be symmetrized in fast and slow axis. Several concepts have been developed for beam shaping of diode laser arrays in the past. State of the art solutions use refractive or reflective optics to cut the emission in slow axis in several sections with subsequent rearranging in fast axis. Because of the high divergence angle in the fast axis direction, all these approaches dispose a collimating lens for fast axis collimation at very short distances from the emitting facet of the laser diodes, i.e. make use of collimating lenses of short focal length for fast axis collimation. This approach usually requires precise alignment of multiple micro-optical lenses in six axes, which makes the whole setup relatively complex and expensive. Nevertheless, a substantial loss of beam quality is experienced because of unavoidable tolerances in the parts, such as smile of the lens or diode array or the optical components for beam shaping, as well as in the alignment of the laser light emitter(s) to the associated optical component(s) and the optical components themselves.
Laser diode arrays of the above kind are disclosed e.g. by US 2003/0048819 A1, US 2004/0114648 A1, U.S. Pat. No. 5,715,264 and U.S. Pat. No. 5,099,488.
In the following a high power laser diode, which forms the basis of the subject-matter of this application and is disclosed in EP 1 830 443 A1 and U.S. Ser. No. 10/778,806 of the applicants, will be described with reference to FIG. 1a to 3. As shown in FIG. 1a, the high power laser diode comprises a common heat sink 106 and a planar alignment substrate 110. The laser diodes 102 are mounted on submounts 101 which are mounted onto the top surface of the common heat sink 106. As shown in FIG. 1b, cut-outs formed by a central cut-out portion 114, a left cut-out portion 115 and a right cut-out portion 116 are formed in the alignment substrate 110, said cut-out portions being formed as through-holes in the alignment substrate 110. As shown in FIG. 1a, the laser diodes 102 mounted onto the submounts 101 are fully received by the cut-outs 114-116 of the alignment substrate 110. As shown in FIG. 1b, the alignment substrate 110 comprises stops 119 formed as edges of the cut-outs 114-116. These stops 119 enable a precise alignment of the laser diodes 101 and/or submounts 102 as will be described in more detail with reference to FIG. 4a to 5b below.
As shown in FIG. 1a, mirrors 107 are unitarily provided on the upper surface of the heat sink 106 for deflecting the output laser beams by an angle of 90 degrees to a direction of light propagation designated by z′ in the following (cf. FIG. 2a). The deflected light beams pass through the right cut-out 116 provided in the alignment substrate 110 near the front facet of the respective laser diode 102.
As shown in FIG. 2a, a planar spacer substrate 130 of a transparent material, such as glass, is bonded onto the top surface of the alignment substrate 110. Further, the fast axis collimating lenses 133, optionally made as an array, are bonded onto the top surface of the spacer substrate 130. For this purpose the fast axis collimating lenses 133 are formed as plano-convex lenses, the rear side of which being bonded onto the top surface 301 of the spacer substrate 130. The spacer substrate 130 is used for precisely defining the distance between the light emitting facets of the laser diode chips 102 and the downstream fast axis collimating lenses 133. Once a proper orientation and alignment of the lens array has been found, this orientation and alignment is fixed by bonding the lens array 133 to the spacer substrate 130. This high power laser diode as described above emits a plurality of fast axis collimated output laser beams, which propagate in the same direction (z′) at equidistant spacings between neighbouring output laser beams and are aligned along a single line.
The set-up relies on highly accurate placement of the diode laser on the heat sink in six axis, which is practically very challenging. Based on an individual alignment of the FAC lens, the assembly tolerances for the diode laser can be significantly reduced for four axis and by a proper design of the beam size of the collimated beam.
FIG. 3a shows a setup for superposing two fast axis collimated output laser beams of high power laser diodes according to FIGS. 2a and 2b each with an optical fill factor of 50% and for subsequent slow axis collimation as disclosed in EP 1 830 443 A1 and U.S. Ser. No. 10/778,806 of the applicant(s). Two high power laser diode modules configured as outlined above are arranged perpendicular to each other. While the output laser beam of the module on the left-hand side passes the beam superposition prism 136 substantially unaffected, the output laser beam of the module on the right-hand side is reflected by the slanted surface of prism 136 such that the two output laser beams are superposed with collinear optical axes. Downstream of prism 136 there is provided a common collimating lens 137 for slow axis collimation to generate a fast and slow axis collimated output laser beam 138. Thus, two rows of laser diode emitters are interleaved and the output laser beams superposed. For this purpose, a fill rate of approx. 50% is used to enable an output laser beam in the shape of a long line, but with identical divergences in slow and fast axis of the collimated beam. The superposition of two output laser beams as shown in FIG. 3a induces further optical losses through inaccuracies of the optical inteleaver and alignment tolerances of the individual rows.
U.S. Pat. No. 6,771,686 B1 discloses a high power laser diode array comprising several laser diode bars with an associated fast axis collimator which is segmented in fast axis direction and a slow axis collimator. The planes of the diode laser bars are offset relative to each other by half a beam pitch. An optical element or a coupling element, which is located between the slow axis collimators and a focussing optics, the beams of the emitters are interleaved with each other to obtain a filling factor of 100%. This arrangement needs an optical interleaver for interleaving the output laser beams.
US 2005/0069260 A1 discloses a setup for combining a plurality of laser beams using interleaved optical plates.